# US 9,038,483 B2 · Wireless passive radio-frequency strain and displacement sensors

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- Number: US 9,038,483 B2
- Document ID: US9038483B2
- Status: Granted
- Kind: B2
- Application number: 13/394,490
- Filed: 2010-09-07
- Published: 2012-11-29
- Granted: 2015-05-26
- Earliest claimed: 2009-09-08
- Assignees: University of Massachusetts
- Applicants: Ramaswamy Nagarajan; Jungrae Park; Sharavanan Balasubramaniam; Mario J. Cazeca; Shivshankar Sivasubramanian; Joey Mead; Julie Chen
- Inventors: Ramaswamy Nagarajan; Jungrae Park; Sharavanan Balasubramaniam; Mario J. Cazeca; Shivshankar Sivasubramanian; Joey Mead; Julie Chen
- Web page: https://patentgrep.com/patent/US9038483B2

## CPC

- G01L 1/14: by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators
- G01B 7/16: for measuring the deformation in a solid, e.g. by resistance strain gauge
- G01B 15/06: for measuring the deformation in a solid
- G01D 21/00: Measuring or testing not otherwise provided for
- G01M 5/0041: {by determining deflection or stress}
- G01M 5/0083: {by measuring variation of impedance, e.g. resistance, capacitance, induction}
- G01M 5/0091: {by using electromagnetic excitation or detection}
- G01L 9/007: {using variations in inductance}
- G01L 9/0072: {using variations in capacitance}
- G01L 9/0075: {using a ceramic diaphragm, e.g. alumina, fused quartz, glass}
- Y10T 29/49103: Strain gauge making
- G01L: MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE (weighing G01G)
- G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01D: MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01M: TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- Y10T: TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION

## Abstract

Wireless strain and displacement sensors wirelessly monitor structural health and integrity, and are made by printing inductor-interdigital capacitor sensing circuits on a variety of substrates, including ceramic substrates, with thermally processable conductive inks. Sensors of the invention can be employed to detect strain and displacement of civil structures, such as bridges and buildings. The sensors include sensing elements that are mounted or printed on stiff, inflexible substrates, which prevent the sensing elements from bending, stretching, or otherwise warping when the sensor is strained. An interlayer between the sensing elements allows the sensing elements to move with respect to each other during application of strain. Thus, strain causes the sensing elements to move but not to deform, causing changes in sensor resonance that can be detected through wireless radio-frequency interrogation. Because the sensing elements do not change shape when under strain, the sensor can undergo millions of measurement cycles before breaking.

## Claims

### Claim 1

A sensor comprising:
(a) a first sensing layer including a plurality of first sensing elements on a first rigid substrate;
(b) a second sensing layer including a second sensing element on a second rigid substrate; and
(c) an electrically insulating interlayer disposed between the first and second sensing layers,
wherein the plurality of first sensing elements and the second sensing element are electromagnetically coupled to each other, wherein a resonance frequency of the sensor varies as a function of relative displacement between the first and second sensing layers, and wherein relative movement between the first and second sensing layers causes a change in a plurality of resonance frequencies associated with the plurality of first sensing elements, thereby directing movement of the first sensing layer relative to the second sensing layer in a plane partitioning the first and second sensing layers.

### Claim 2 (depends on 1)

The sensor of claim 1, wherein at least one of the first and second sensing elements includes an electrically conductive material.

### Claim 3 (depends on 1)

The sensor of claim 1, wherein at least one of the first and second sensing layers is formed by an additive technique.

### Claim 4 (depends on 3)

The sensor of claim 3, wherein the additive technique includes screen-printing.

### Claim 5 (depends on 1)

The sensor of claim 1, wherein at least one of the first and second sensing layers is formed by a subtractive technique.

### Claim 6 (depends on 5)

The sensor of claim 5, wherein the subtractive technique includes laser cutting.

### Claim 7 (depends on 1)

The sensor of claim 1, wherein the first and second rigid substrates inhibit deformation of the first and second sensing elements, respectively, during relative motion of the first sensing layer with respect to the second sensing layer.

### Claim 8 (depends on 1)

The sensor of claim 1, wherein at least one of the first and second rigid substrates includes at least one member of a group consisting of glass, ceramics, thermoset Bakelite, composite materials, wood, cement, and concrete.

### Claim 9 (depends on 1)

The sensor of claim 1, wherein the interlayer is an adhesive interlayer.

### Claim 10 (depends on 9)

The sensor of claim 9, wherein the adhesive interlayer includes at least one member of a group consisting of thermoplastics, elastomers, and curable adhesives.

### Claim 11 (depends on 1)

The sensor of claim 1, wherein the interlayer includes air.

### Claim 12 (depends on 1)

The sensor of claim 1, wherein the interlayer has a fatigue life of at least one million measurement cycles.

### Claim 13

A method of sensing strain, the method comprising the steps of:
(a) transmitting a radio frequency (RF) signal toward a sensor to produce a transmitted or reflected RF signal, wherein the sensor comprises a first sensing layer including a plurality of first sensing elements on a first rigid substrate, a second sensing layer including a second sensing element on a second substrate, and an insulating interlayer disposed between the first and second sensing layers, wherein the plurality of first sensing elements are electromagnetically coupled to the second sensing element, and wherein relative movement between the first and second sensing layers causes a change in a plurality of resonance frequencies associated with the plurality of first sending elements, thereby detecting direction of movement of the first sensing layer relative to the second sensing layer in a plane partitioning the first and second sensing layers; and
(b) detecting the transmitted or reflected RF signal, wherein a center frequency of a resonant peak in the reflected or transmitted RF signal indicates strain exerted on the sensor.

### Claim 14 (depends on 13)

The method of claim 13, wherein the RF signal is transmitted wirelessly toward the sensor.

### Claim 15 (depends on 13)

The method of claim 13, wherein the RF signal is transmitted along a path other than a line-of-sight path.

### Claim 16 (depends on 13)

The method of claim 13, wherein at least one of the first and second sensing layers includes an electrically conductive material.

### Claim 17 (depends on 13)

The method of claim 13, wherein the first and second rigid substrates inhibit deformation of the first and second sensing layers, respectively, during movement of the first sensing layer with respect to the second sensing layer.

### Claim 18 (depends on 13)

The method of claim 13, wherein the interlayer deforms during movement of the first sensing layer with respect to the second sensing layer.

### Claim 19 (depends on 13)

The method of claim 13, wherein the interlayer includes air.

### Claim 20 (depends on 13)

The method of claim 13, wherein an absence of the resonant peak indicates formation of a crack in at least one of the upper and lower sensing elements.

### Claim 21

A method of making a sensor, the method comprising the steps of:
(a) forming a first sensing layer including a plurality of first sensing elements on a first rigid substrate;
(b) forming a second sensing layer including a second sensing element on a second rigid substrate; and
(c) disposing an electrically insulating interlayer between the first and second sensing layers,
wherein the plurality of first sensing elements and the second sensing element are electromagnetically coupled to each other, wherein the first and second rigid substrates inhibit deformation of the first and second sensing layers, and wherein relative movement between the first and second sensing layers causes a change in at least a plurality of resonance frequencies associated with the plurality of first sensing elements, thereby detecting direction of movement of the first sensing layer relative to the second sensing layer in a plane partitioning the first and second sensing layers.

### Claim 22 (depends on 21)

The method of claim 21, wherein forming at least one of the first and second sensing layers includes the step of additive application of the respective sensing element to the respective rigid substrate.

### Claim 23 (depends on 21)

The method of claim 21, wherein forming the first sensing layer includes depositing an electrically conductive pattern on the first rigid substrate.

### Claim 24 (depends on 21)

The method of claim 21, wherein forming at least one of the first and second sensing layers includes the step of subtracting material from electrically conductive material.

### Claim 25 (depends on 21)

The method of claim 21, wherein forming the first sensing layer includes laser cutting a metallic element.

### Claim 26 (depends on 21)

The method of claim 21, wherein disposing the interlayer includes adhering the interlayer to at least one of the first and second sensing layers.